Induction of embryonic diapause in the calanoid copepod Acartia hudsonica

Abstract

The copepod Acartia hudsonica grows and reproduces in Narragansett Bay in winter and spring, spending summers as dormant eggs. I investigated the change in egg types from immediately-hatching eggs to dormant eggs. Temperature and length of day were hypothesized to act as the cues inducing this population to produce dormant eggs, and the variation in the dormant-egg responses of individual females was quantified. A. hudsonica produced some dormant eggs early in the spring, but most of the switch to dormant eggs occurred in June. Experiments showed that temperatures above about 16°C increased the proportion of dormant eggs produced whereas increased length of day did not. In several experiments between 42% and 85% of the variance in the fraction of dormant eggs produced was attributed to variance among individuals. ^ In estuaries in the northern part of its range, A. hudsonica populations are present year-round and may not express diapause. This difference may be genetic or phenotypic. Using a full-sib rearing design and analysis of variance, I assessed the relative importance of these two sources of variation in two populations, northern and southern. Both populations showed phenotypic plasticity in fraction of dormant eggs produced. Further, both populations showed significant sibship-environment interactions. Both populations also revealed a significant amount of genetic variation in the fraction of dormant eggs produced. ^ It has been hypothesized that a storage bank of viable embryos in diapause can act to maintain genetic variation in a population by harboring genotypes that were successful in the past. If so, we may be able to detect changes in gene frequencies that correspond to diapause induction or termination. I hypothesized that allozyme allele frequencies would reflect changes in the genetic variability of two copepod populations in relation to the induction of seasonal diapause or the seasonal emergence of nauplii from eggs in diapause. Acartia tonsa and A. hudsonica had distinct patterns of enzyme migration at common loci. Further, A. tonsa showed significant changes in allele frequencies at all loci examined, while A. hudsonica showed changes at Mpi only. However, there were no systematic temporal or allelic patterns to the allele frequency changes. ^